The present invention relates to a process for increasing the rate and temperature of crystallization of molding compositions of thermoplastic polyesters such as polyethylene terephthalate (PET) and to molding compositions of thermoplastic polyesters having increased crystallization rates and temperatures. In particular, the present invention relates to molding compositions of thermoplastic polyesters to which a quaternary ammonium salt has been added as a nucleating agent, which nucleating agent increases the crystallization rate and temperature of the polyester.
PET is widely used in the manufacture of molded plastics, films and fibers, because of its excellent physical properties However, the mold processing of PET is inefficient because it has a slow rate of crystallization, requiring high mold processing temperatures and long mold cycling times
The crystallization rate and temperature of PET can be increased by the addition of a nucleating agent to a molding composition. Candidate nucleating agents should promote rapid crystallization under conditions of rapid cooling from the melt. The nucleating agent should be highly effective at low concentration in promoting crystallization, and readily dispersible so that rapid and uniform crystallization of the polymer from the melt is possible If the crystallization of PET is slow and if the crystallization process during injection molding is substantially incomplete, then the dimensional stability of the molded parts will be poor. Also, because of the slow crystallization rate of PET, the injection moldability of PET is very poor.
Commonly used nucleating agents are the alkali or alkaline earth metal salts of organic acids For example, U.S. Pat. No. 3,352,904 to Deyrup discloses the use as PET nucleating agents of sodium or potassium salts of carboxylic acids or the salts of organic polymers containing pendant carboxyl groups, including carboxylic acid groups. The use of ionizable metal salts of aromatic rings substituted with acid groups selected from carboxylic acid, sulphinic acid, phosphonic acid, phosphinic acid and phenol groups are disclosed in U.S. Pat. Nos. 4,322,335 and 4,368,288 to Nield, U.S. Pat. No. 4,305,864 to Griffin, U.S. Pat. No. 4,390,493 to Yasui, U.S. Pat. No. 4,393,178 to Legras, EP 21,648 to Biebuyck and EP 351,732 to Karasawa.
U.S. Pat. No. 3,575,931 to Sherman discloses the use of metal salts of carboxylic acids as PET nucleating agents. U.S. Pat. No. 4,401,792 to Axelrod discloses the use of alkali metal salts of polyethylene ionomers and benzoic acid as PET nucleating agents. U.S. Pat. No. 4,425,470 to Garcia discloses the use of alkali metal salts of ethylene terephthalate oligomers as nucleating agents for PET. U.S. Pat. No. 4,551,507 to Haylock discloses the use of alkali or alkaline earth metal carboxylate salts of tertiary or quaternary amines as PET nucleating agents.
U.S. Pat. No. 4,390,493 cited above notes that these nucleating agents are salts of either weak or strong acids. The salts of strong acids cannot be dispersed uniformly in PET, although their crystallization acceleration effect is great. This causes nonuniform crystallization, which, as discussed above, reduces the impact resistance and other properties of articles molded from the PET. While salts of weak acids disperse more uniformly through the PET, their crystallization acceleration effect is poor, requiring large quantities of these nucleating agents to be added to the polymer, which also hinders the uniform dispersion of these nucleating agents and consequently the uniformity of the crystallization of the polymer.
A PET nucleating agent that was not a metal salt of an organic acid would be highly desirable, so that the nucleating agent would be capable of being more uniformly dispersed in the PET.